1. Field of the Invention
The present invention relates to a surface emitting laser, a manufacturing method of the surface emitting laser, a surface emitting laser array, a manufacturing method of the surface emitting laser array, and an optical apparatus including the surface emitting laser array.
2. Description of the Related Art
As one of the surface emitting lasers, there is known a vertical cavity surface emitting laser (hereinafter referred to as VCSEL).
According to the surface emitting laser of this type, a light beam can be taken out perpendicularly to a semiconductor substrate surface, and hence a two-dimensional array can be easily formed only by changing a mask pattern at the time of forming the laser elements.
The parallel processing using the plurality of beams emitted from the two-dimensional array enables the increase in density and speed, and hence is expected to be applied in various industrial fields.
For example, when the surface emitting laser array is used as an exposure light source of an electrophotographic printer, it is possible to increase the density and speed in the printing process by using the plurality of beams.
In such electrophotographic printing process, it is necessary to form stable and minute laser spots on a photosensitive drum. Thus, a stable operation in a single transverse mode or a single longitudinal mode is also required as a laser characteristic.
In recent years, there has been developed a method in which a current constriction structure is formed so as to allow current to be injected only into a necessary region.
In this method, in order to enhance the performance of the surface emitting laser, current is allowed to be injected only into a necessary region by forming the current constriction structure in such a manner that a layer having a high Al composition, for example, an AlGaAs layer having an Al composition of 98% is formed in a multilayer film reflection mirror and selectively oxidized in a high temperature steam atmosphere.
Meanwhile, the above described method using selective oxidization is not desirable from the viewpoint of realizing a single transverse mode oscillation.
That is, a refractive index difference which is larger than needed is caused due to the existence of the oxidized layer, so that high order transverse modes are generated.
As a method to cope with this problem, there is used a method, and the like, in which a single transverse mode oscillation is achieved in such a manner that the diameter of the light emitting region is reduced to about 3 μm by using the above described current constriction structure so as to prevent the high order transverse modes from being confined.
However, in the method of restricting the light emitting region, the light emitting region is reduced, and thereby the output per element is significantly reduced.
For this reason, heretofore, there have been investigated methods for realizing a single transverse mode oscillation while maintaining a light emitting region which is larger to some extent than the region obtained in the case where the single transverse mode oscillation is realized only by reducing the light emitting region by using the above described current constriction structure.
That is, there have been investigated methods in which a single transverse mode oscillation can be realized by intentionally introducing a loss difference between a fundamental transverse mode and a high order transverse mode, while maintaining a light emitting region that is large to some extent.
As one of the methods, a so-called surface relief method is disclosed in Japanese Patent Application Laid-Open No. 2001-284722, H. J. Unold et al., Electronics Letters, Vol. 37, No. 9 (2001) 570, and J. A. Vukusic et al., IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 37, No. 1, 2001 (108).
The surface relief method is a method in which the loss in the high order transverse mode is increased to be larger than the loss in the fundamental transverse mode by applying level difference processing to the element surface which is the light emitting surface of the surface emitting laser element.
Note that in the present specification, it is assumed that, in the following, the level difference structure, which is provided, as described above, in the light emitting region of the light emitting surface of the reflection mirror to control the reflectance of the reflection mirror, is referred to as a surface relief structure.
Generally, as a mirror for the VCSEL, there is used a multilayer film reflection mirror in which a plurality of pairs of layers, each of which has a different refractive index and an optical thickness of one fourth of a laser oscillation wavelength (that hereinafter may be referred to as ¼ wavelength unless otherwise specified), are laminated so that the two layers are alternately arranged.
Usually, the multilayer film reflection mirror is terminated by a high refractive index layer, so that a high reflectance of 99% or more is obtained by also using the reflection on the final interface with air having a low refractive index.
Here, there will be first described a convex surface relief structure illustrated in FIG. 2A. Such convex surface relief structure is also disclosed in H. J. Unold et al., Electronics Letters, Vol. 37, No. 9 (2001) 570. When as in a low reflection region 204 illustrated in FIG. 2A, the final layer of a high refractive index layer 206 (having an optical thickness of ¼ wavelength) is removed, the multilayer film reflection mirror is terminated by a low refractive index layer 208. Thereby, a convex surface relief structure is obtained.
According to such convex surface relief structure, the phase of a light beam reflected by the interface between the low refractive index layer 208 and the air having a refractive index lower than that of the low refractive index layer 208 is made to shift by π from the phase of a light beam which is totally reflected by the multilayer film reflection mirror that exists under the interface.
For this reason, in the low reflection region 204, the reflectance is reduced to, for example, 99% or less, so that the reflection loss can be increased to about 5 to 10 times.
The above principle is used in order to introduce a loss difference between the fundamental transverse mode and the high order transverse mode. That is, the low reflection region 204 is formed only in the peripheral portion of the light emitting section so as to increase the overlap between the low reflection region 204 and a high order transverse mode light distribution 212.
On the other hand, a high reflection region 202 is left in the central portion of the light emitting section so as to increase the overlap between a fundamental transverse mode light distribution 210 and the high reflection region 202 in which the final layer of the high refractive index layer 206 is left.
Thereby, the reflection loss in the high order transverse mode is increased, so that the high order transverse mode oscillation is suppressed and only the fundamental transverse mode oscillation is obtained.
Further, as in the low reflection region 204 illustrated in FIG. 2B, it is also possible to configure a concave surface relief structure in such a manner that a low refractive index layer is (or a high refractive index layer may also be) further added on the final layer of the high refractive index layer 206 by an optical thickness of ¼ wavelength.
Such concave surface relief structure is also disclosed in Japanese Patent Application Laid-Open No. 2001-284722. Even with such configuration, the phase is shifted by π in the low reflection region 204, and hence the reflectance can be reduced based on the same principle as the convex surface relief structure, so that the fundamental transverse mode oscillation can be realized.
According to the above described surface relief structure in the prior art form disclosed in Japanese Patent Application Laid-Open No. 2001-284722, H. J. Unold et al., Electronics Letters, Vol. 37, No. 9 (2001) 570, and J. A. Vukusic et al., IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 37, No. 1, 2001 (108), a single transverse mode oscillation can be realized while the light emitting region is maintained to be larger to some extent than the light emitting region obtained in the case where the single transverse mode is realized only by the current constriction structure.
However, the reflectance in the surface relief structure is sensitively influenced by the thickness of layers forming the surface relief structure, and has a great influence on realizing the single transverse mode oscillation. Therefore, it is extremely important to control the layer thickness in the manufacture of the surface relief structure.
That is, the surface relief structure has a feature that the reflectance (reflection loss) is very sensitive to the layer thickness removed or added in the manufacture of the surface relief structure.
Next, there will be further described features of the above described surface relief structure.
FIG. 3 is a figure which is described in J. A. Vukusic et al., IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 37, No. 1, 2001 (108) and in which the layer thickness removed by the surface relief is plotted along the horizontal axis while the induced loss is plotted along the vertical axis (left side).
From FIG. 3, there can be seen a state where the loss has peaks appearing periodically with respect to the layer thickness which is removed.
Further, the peak is steep, and when a desired loss value is to be introduced, the layer thickness which is removed needs to be controlled with very high accuracy (+/−5 nm or less).
On the other hand, the amount of the loss greatly influences the extent to which the single mode oscillation is realized, and further greatly influences the output characteristic of the element.
Therefore, in the manufacture of the surface relief structure, it is necessary to highly precisely control the layer thickness in order to manufacture elements which have good reproducibility and uniformity and which are capable of performing a single mode operation with the same characteristics.
In other words, it can be said that in the case where the surface relief structure is manufactured, when the thickness of the layer to be removed or added can be simply grasped with high accuracy and when the layer thickness can be adjusted according to the grasped layer thickness, the reproducibility and uniformity in the manufacture of the element can be greatly improved.
An object of the present invention is to provide a surface emitting laser including a convex surface relief structure, the layer thickness of which can be highly precisely controlled and which is capable of performing a single mode operation with good reproducibility and uniformity, and to provide a manufacturing method of the surface emitting laser.